There are several different form factors (or “standards”) for PC power supplies. These form factors define not only the physical size of the power supply, but also the kind of connectors the power supply has. As of this writing ATX12V 2.x and EPS12V are the most common standards for PC power supplies.

AT: This standard was introduced by the IBM PC AT in 1984 and was used until the ATX standard gained popularity in the mid-1990’s. This power supply delivers four voltages, +5 V, +12 V, -5 V and -12 V, and the main motherboard connector uses a 12-pin connector (see previous page). From the connectors presented this kind of power supply uses only the standard peripheral power connectors and the floppy disk drive power connector, in addition to the 12-pin motherboard cable.

ATX: In 1996, Intel introduced a new motherboard layout called ATX to replace the old AT layout. Because the ATX motherboard had completely different physical dimensions, new cases were also needed (“ATX cases,” in contrast to the “AT cases” used so far). With this new motherboard layout, Intel also proposed a new kind of power supply providing new features, such as the use of a new 20-pin motherboard connector and the introduction of two new voltages, +3.3 V and +5VSB, also known as “standby power.” This output is always turned on, even when the computer is turned off, which allows the computer to turn itself off without requiring you to press an on/off switch. From the connectors presented, this kind of power supply uses only a 20-pin motherboard connector, the standard peripheral power connectors and the floppy disk drive power connector. You can find the complete ATX specification here.

ATX12V 1.x: With modern CPUs requiring more power, two extra connectors were added to ATX power supplies: a four-pin 12 V connector (ATX12V connector) and a six-pin auxiliary power (see previous page). ATX12V 1.3 introduced the Serial ATA power connector. You can find the complete ATX12V 1.x specification here.

ATX12V 2.x: This form factor, introduced with the release of the PCI Express bus, upgraded the main motherboard power connector to a 24-pin model (Figures 6 and 7) and introduced the PCI Express auxiliary power connector (PEG, Figures 13 and 14). You can find the complete ATX12V 2.x specification here. This is the standard used nowadays.

EPS12V: This form factor was created by SSI (Server System Infrastructure) for entry-level servers. Its current version uses the same plugs used on the ATX12V 2.x, adding a new CPU power plug, called EPS12V (see Figures 10, 11 and 12). Since it brings only one new connector, many power supply manufacturers provide models that are ATX12V v2.x and EPS12V at the same time. You can find the complete EPS12V specification here.

So far we took a look at the main power supply form factors for desktop PCs. There are, however, other form factors available for small form factor PCs.

CFX12V: CFX stands for Compact Form Factor. It uses the same connectors as ATX12V v2.x and is “L” shaped based on the standard ATX size, with a 5.90” (150 mm) width at its top and 4” (101.6 mm) width at its bottom. You can find the complete CFX12V specification here.